The Domain Name System

1
The Domain Name System

• Outline
• Domain Name System

2
Domain Name System Overview

• What are names used for in general?
• identify objects
• locate objects
• define membership in a group
• Basic Terminology
• Name space
• defines set of possible names
• Consists of a set of name to value bindings
• Resolution mechanism
• When invoked with a name returns corresponding
  value

3
DNS Properties

• Size of Internet demands well devised naming
  mechanism
• Specified in RFC 1034, 1035 (Mockapetris 87)
• Names versus addresses
• Human readable versus router readable
• Location transparent versus location-dependent
• Flat versus hierarchical
• Can names be divided into components?
• Global versus local
• What is the scope of naming?
• DNS for other purposes
• Determines where user requests are routed

4
Examples

• Hosts
• pluto.cs.wisc.edu 192.12.69.17
• 192.12.69.17 8023A8335B9F
• Files
• /usr/llp/tmp/foo (server, fileid)
• Users
• Paul Barford pb_at_cs.wisc.edu

5
Examples (cont)

• Mailboxes
• Services
• nearby ps printer with short queue and 2MB

6
Domain Naming System

• Hierarchical name space for Internet objects
• Names are read from right to left separated by
  periods
• Each suffix in a domain name is a domain
• wail.cs.wisc.edu, cs.wisc.edu, wisc.edu, edu

7
Name Servers

• Partition hierarchy into zones (administrative
  authorities)

edu
com
gov
mil
org
net
uk
fr










princeton
mit
cisco
yahoo
nasa
nsf
arpa
navy
acm
ieee
cs
ee
physics
ux01
ux04

• Each zone implemented by two or more name servers

8
Resource Records

• Each name server maintains a collection of
  resource records
• (Name, Value, Type, Class, TTL)
• Each record is a translation based on type
• Name/Value not necessarily host names to IP
  addresses
• Type (some examples)
• A Name full domain name, Value IP address
• NS Value gives domain name for host running name
  server that knows how to resolve names within
  specified domain.
• CNAME Value gives canonical name for particle
  host used to define aliases.
• MX Value gives domain name for host running mail
  server that accepts messages for specified
  domain.
• Class allow other entities (other than NIC) to
  define types
• IN is what is used by the Internet
• TTL how long the resource record is valid

9
Root Server

• (princeton.edu, cit.princeton.edu, NS, IN)
• (cit.princeton.edu, 128.196.128.233, A, IN)
• (cisco.com, thumper.cisco.com, NS, IN)
• (thumper.ciscoe.com, 128.96.32.20, A, IN)

10
Princeton Server

• (cs.princeton.edu, optima.cs.princeton.edu, NS,
  IN)
• (optima.cs.princeton.edu, 192.12.69.5, A, IN)
• (ee.princeton.edu, helios.ee.princeton.edu, NS,
  IN)
• (helios.ee.princeton.edu, 128.196.28.166, A, IN)
• (jupiter.physics.princeton.edu, 128.196.4.1, A,
  IN)
• (saturn.physics.princeton.edu, 128.196.4.2, A,
  IN)
• (mars.physics.princeton.edu, 128.196.4.3, A, IN)
• (venus.physics.princeton.edu, 128.196.4.4, A, IN)

11
CS Server

• (cs.princeton.edu, optima.cs.princeton.edu, MX,
  IN)
• (cheltenham.cs.princeton.edu, 192.12.69.60, A,
  IN)
• (che.cs.princeton.edu, cheltenham.cs.princeton.edu
  , CNAME, IN)
• (optima.cs.princeton.edu, 192.12.69.5, A, IN)
• (opt.cs.princeton.edu, optima.cs.princeton.edu,
  CNAME, IN)
• (baskerville.cs.princeton.edu, 192.12.69.35, A,
  IN)
• (bas.cs.princeton.edu, baskerville.cs.princeton.ed
  u, CNAME, IN)

12
Name Resolution

• Strategies
• forward
• iterative
• recursive
• Local server
• need to know root at only one place (not each
  host)
• site-wide cache

13
DNS Issues

• Top level domain names are tightly controlled
• Before an institution is granted authority for a
  second-level domain, it must agree to operate a
  DNS server that meets Internet standards.
• Eg. all DNS info must be replicated on separate
  systems
• DNS is very important in the Internet
• Security of this system is strict
• DNS lookups can affect performance
• In practice DNS is much more complicated than you
  might think

14
DNS Redirection and CDNs

• Up to now, we have assumed that there is a single
  mapping between a name and an IP
• Content delivery companies (Akamai) use DNS to
  direct client requests to mirror servers
• Content Delivery Networks (CDNs) attempt to push
  content closer to the edge of the network
• Distributed network of mirror servers
  (caches/proxies)
• How do clients find the closest mirror?
• CDNs take over companys name server

15
DNS Redirection contd.

• Local DNS request gets routed to companys name
  server
• CDN assumes client is near their local DNS
• CDN responds with IP of server which is closest
  to clients local DNS
• Enables much
• Makes many assumptions

16
Other Naming Protocols

• X.500
• Naming system designed to identify people
• Each person is defined by attributes
• Name
• Title
• Too cumbersome
• Lightweight Directory Access Protocol (LDAP)
• Evolved from X.500
• System for learning about users